1. Field of the Invention
The present invention pertains to novel silsesquioxane polymer precursors, to methods of preparing them, and to ultraviolet transmissive polymers prepared therefrom.
2. Background Art
Transparent polymers are widely used. Polymers exhibit distinct processing advantages over other transparent media such as glasses, including fused or vitreous silica, and crystalline substances such as calcium fluorite. Polymers may be thermoplastic, i.e. melt processable, or may be thermoset, i.e. curing to a non-thermoplastic state by means of crosslinking. Hybrid polymers, for example those which are initially thermoplastic but which further crosslink upon heating, exposure to light, or over time, are also available.
From a process standpoint, organic based materials are desirable since they are compatible with low-temperature processing. Accordingly, the development of transparent materials that possess both advantages of organic and inorganic materials is becoming important. In other words, materials that have high transparency and durability to short wavelength light (advantages of inorganic materials) and ease of processing at low temperatures (advantages of organic materials) are needed.
Many applications require polymers which are robust, which are abrasion resistant, which have low coefficients of thermal expansion and refractive index, and which are transmissive over the required portion of the spectrum. Ordinary glass, for example, is transmissive over the visible region and partially into the infrared (IR) and ultraviolet (UV) regions of the spectrum. However, for emissive or responsive devices which operate further in the IR or UV, quartz or fused silica must be used. These materials are difficult to fabricate.
Polymers previously used for such applications, such as transparent epoxy resins, polyesters, polyacrylates, and organopolysiloxanes may have unwanted absorbtion peaks in the spectral areas of interest due to their chemical linkages and the presence of absorbing groups. Many of these materials do not transmit appreciably in the IR and/or UV areas of the spectrum. Importantly, these polymers also suffer from lack of abrasion resistance, and have undesirably high coefficients of thermal expansion and refractive index. Thus, while being quite processable, their optical and physical properties are less than desired.
Silsesquioxanes as polyhedral structures are generally known. Silsesquioxanes may be prepared by hydrolysis of trifunctional silanes such as A3SiB where A is an alkoxy group or halogen and B is a functional or non-functional group, for example an alkyl group, vinyl group, or hydrogen. While polyhedral molecules can result from conventional synthetic methods, the primary products are generally highly crosslinked resins which are soluble in apolar solvents such as toluene. The resins may be cured by reaction with interreactive functional monomers to produce a wide variety of products. For example, Si—H functional resins may be cured with divinyl compounds. However, the products are generally high molecular weight elastomers or solids, and often contain numerous unreacted functional groups, even when reacted in solution.
Resinous silsesquioxanes are known from U.S. Pat. No. 5,047,492, and have been cited as of academic interest as well. Some of these products may contain polyhedral silsesquioxanes. However, when polyhedral silsesquioxanes are functionalized directly at a cage silicon atom, i.e. when functional groups R in FIG. 1 are vinyl or H, the spatial arrangement of functional groups and their close proximity to the inflexible cage renders their reaction incomplete. This is particularly the case since prior functionalized silsesquioxanes have been completely functionalized, i.e. in silsesquioxane cages containing eight silicon atoms, each silicon bears the same reactive functional group. Unreacted groups can later react, altering the properties of the product, or may enter into a variety of degradation reactions, for example oxidizing to produce colored species or hydrolyzing to produce hydrolysis products as inclusions, which decreases transparency.
It would be desirable to provide easily processable polymers which do not have the above identified deficiencies, all or in part.